Abstract

Background

The aim of this study was to examine potential therapeutic effect of the two NO donors NCX 2057 (3-(4-hydroxy-3-methoxyphenyl)-2-propenoic acid) 4-(nitrooxy)butyl ester) and SNP (sodium nitroprusside) on the early allergic airway response in the peripheral lung.

Methods

The experiments were performed in guinea pig lung parenchyma (GPLP) derived from ovalbumin (OVA) sensitized guinea pigs. The effects of NCX 2057 and SNP were evaluated by contractile responses and mediator release during OVA challenge. The generation of nitrite and nitrate was assessed by chemiluminescence. Statistical analysis was evaluated by ANOVA.

Conclusion

Although both SNP and NCX 2057 reduced the release of pro-inflammatory mediators, their profiles were distinctly different. Furthermore, NCX 2057 also induced smooth muscle dilation in the GPLP. The findings point to specific anti-inflammatory effects of different NO donors in the peripheral lung tissue.

Keywords

Nitric OxideHistamineFerulic AcidCetirizineLTB4

Background

Administration of exogenous nitric oxide (NO) and nitro vasodilators has received considerable attention, mainly due to their therapeutic ability and haemodynamic effects, and are well established drugs for treatment of cardiac disorders [1, 2]. Exogenous NO also has the ability to exert bronchodilatory effects in bronchial asthma [3] and NO is used in the treatment of preterm children to improve lung capacity [4]. However, the effect of NO donors during the early allergic airway response requires further evaluation, especially in the distal lung.

In the peripheral lung, the release of histamine and eicosanoids (leukotrienes and prostaglandins) from activated inflammatory cells, such as mast cells and macrophages, may contribute significantly to the symptoms of allergic rhinitis and asthma [5–8]. In the airways, mast cells and alveolar macrophages also represent a major source of NO [2, 9–11], which may act both directly on smooth muscle cells and in an autocrine fashion to suppress allergen-induced responses, as release of histamine [12] and leukotriene synthesis [13]. Inhibitors of NO synthases have been shown to enhance antigen-induced bronchoconstriction in sensitized guinea pigs by increased generation of leukotrienes [14].

The aim of this study was therefore to examine the role of NO donors in antigen-induced responses in the peripheral part of the lung. Thus, two structurally different NO donors, NCX 2057 (3-(4-Hydroxy-3-methoxyphenyl)-2-propenoic acid) 4-(nitrooxy) butyl ester) [15] and SNP (sodium nitroprusside; Na2 [Fe(CN)5NO]*2H20) were used (fig 1). The substance NCX 2057 (fig 1a) belongs to a newly class of developed NO donors that are chemically conjugated to a variety of therapeutic drugs, including the anti-histamine cetirizine [15] and NSAIDs [16]. NCX 2057 has also been shown to have anti-inflammatory properties [17]. Therefore, the parent compound of NCX 2057, ferulic acid, was also studied, as this substance has been described to have anti-inflammatory potential [18]. The other NO donor, SNP (fig 1b), is a vasodilator, used in cardiovascular treatments to lower blood pressure or to improve cardiac function [19].

Figure 1

Chemical structure of the NO donors A, NCX 2057 and B, SNP. The arrow indicates cleavage site of NO conjugation on substance NCX 2057.

The experiments were performed in actively sensitized guinea pig lung parenchyma (GPLP), an in vitro model for antigen-induced contractions [20]. Recent studies of antigen-induced airway constriction in the guinea pig lung indicate that the responses to allergen in humans are similar to the responses obtained in the guinea pig [21, 22], since histamine, cysteinyl-leukotrienes (CysLTs) and several prostanoids contribute to the antigen-induced airway constriction in these particular species. The hypothesis to be tested was that NO donors in the peripheral lung may affect antigen-induced contractions either generally by dilatation of smooth muscle tissue or specifically via actions on release of histamine or eicosanoids. The data revealed that the tested NO donors SNP and NCX 2057 acted as inhibitors of allergen-induced mediator release.

Lung parenchymal strips and organ bath experiments

The animals were sacrificed by an overdose of inhaled CO2 and the heart-lung-package was quickly removed and placed in ice-cold Tyrode's solution (prepared each day, containing NaCl 149.2 mM, KCl 2.7 mM, NaHCO3 11.9 mM, glucose 5.5 mM, CaCl2 1.8 mM, MgCl2 0.5 mM, NaH2PO4 0.4 mM). The lung parenchyma was cut parallel to the peripheral margins, yielding four to eight strips, each having a size of 2 × 2 × 20 mm and a weight of approximately 60 mg. The lung parenchymal strips were set up at a resting tension of 2.5 mN (0.25 g) in 5 ml organ baths filled with Tyrode's solution, bubbled with carbogen gas (6.5% CO2 in O2) to keep a pH of 7.4 and the temperature was kept at 37°C [14, 15]. Changes in smooth muscle tension, contractions and relaxations, were recorded via isometric force-displacement transducers connected to a Grass polygraph and responses were displayed by using the IOX data acquisition system (EMKA, France). Data were analyzed by the software program Dataanalyst (EMKA, France). After an equilibration period of 90 min and washes each 15 min, histamine (1–30 μM) was added cumulatively as a control of the GPLP reactivity. Preparations displaying contraction responses less than 1.0 mN to the highest concentration of histamine were excluded from further experiments. Another wash and equilibration period between histamine and treatment period was performed. All drugs were given 15 min before the challenges.

OVA was added as cumulative challenge of increasing concentrations (1–10,000 ng/ml) every 10 min without changing bath fluid. The cumulative OVA concentrations were chosen to generate a concentration-response curve with no contractions at the lowest dose of 1 ng/ml and maximum contractions at the final dose of 10,000 ng/ml OVA. For study of smooth muscle contractions, the GPLP was contracted with cumulative doses of LTD4 (0.1–100 nM). 10 nM of LTD4 generated a 50% contraction of the GPLP and this EC50 dose was chosen for studies of smooth muscle relaxation. These doses have previously been used in the GPLP in vitro test system [14, 15, 20]. The GPLP was precontracted with LTD4 (10 nM) before cumulative addition of cGMP analogue 8-Bromo-cGMP, SNP or NCX 2057. cGMP-dependent relaxation was evaluated by the addition of the guanylyl cyclase inhibitor ODQ (30 μM), which previously has been demonstrated to inhibit vasodilation induced by NCX 2057 [15]. Maximum contractions of the preparation were determined with histamine (1 mM), acetylcholine (1 mM) and KCl (50 mM) at the end of each experiment, and other responses were expressed as percent of maximum contractions.

Measurements of released mediators with enzyme immunoassays

A 1-mL aliquot of organ bath fluid was collected from each organ bath and immediately frozen at -20°C. The samples were taken at the end of the equilibration period to obtain basal mediator release from the tissue and at the obtained contractile plateau after challenge with OVA 1000 ng/ml or at the obtained contractile plateaus after cumulative doses of OVA (1, 10, 100 and 1000 ng/ml). Enzyme immunoassay (EIA) analyses of the different mediators LTB4, CysLTs and the prostanoids thromboxane (TXA2) and prostaglandin (PG) D2 were performed according to the manufacture's instructions. TXA2 was measured as the stable metabolite TXB2. PGD2 was measured as PGD2-mox. The assay detection limits in the bath fluid levels for the different mediators were 7.8 pg/ml for TXB2, PGD2 and LTE4 and 3.9 pg/ml for LTB4. Results below detection limits were set as zero in the statistical evaluation. The EIA specificity for the different mediators to interfere with each other was less than 0.01%, with the exception of the EIA kit for TXB2 cross reacted with PGD2 (0.53%) and with PGE2 (0.09%). The EIA kit for CysLTs was performed with the CysLT antiserum and the sera cross reacted with LTC4 (100%) and LTD4 (100%) and LTE4 (67%). Histamine was measured as previously described [23, 24]. Duplicates of 300 μl were placed in 96-wells plates and the amount of histamine was analyzed by a fluorospectrometer at the wavelength 450 nM. The detection limit for histamine was 3.9 ng/ml.

Measurements of nitrite and nitrate with chemiluminescence

Aliquots of bath fluid were collected from the organ bath for measurements of nitrite and nitrate to obtain an indirect assessment of the NO releasing profile for the NO donors. To determine the basal production of nitrite and nitrate in the bath fluid, a sample volume of 200 μl was withdrawn from each bath before the addition of any drug. Bath fluid aliquots (200 μl) were then collected at 15, 30, 60, 90 and 120 min after the addition of Tyrode's solution, DMSO, NCX 2057 (100 μM) or SNP (100 μM). The amount of nitrite and nitrate was analyzed with a chemiluminescence method as previously described [25].

Data analysis and statistical procedures

All data are presented as mean ± standard error of the mean (s.e.m.). Statistical analyses were made for paired and unpaired observations by Student's t-test or analyses of variances (ANOVA). All concentration-response curves were statistically analyzed with two-way ANOVA followed by the post hoc tests Tukey's t-test or Bonferroni's t-test. A p-value of less than 0.05 was considered significant.

Drugs and chemical reagents

NaCl, KCl, CaCl2, MgSO4, NaHCO3, KH2PO4 and glucose were obtained from VWR International (West Chester, Pennsylvania, USA). Histamine dihydrochloride, acetylcholine, ovalbumin (OVA, chicken egg albumin, grade II), dimethylsulfoxid (DMSO), ferulic acid, ODQ (1H(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one), 8-bromo-cGMP and SNP were purchased from Sigma-Aldrich (St. Louis, Missouri, USA). LTD4 was from Cascade Biochemicals Ltd. (Reading, UK). The EIA kits for CysLTs, LTB4, TXB2 and PGD2-mox were obtained from Cayman Chemicals (Ann Arbor, Michigan, USA). NCX 2057 (3-(4-hydroxy-3 methoxyphenyl)-2-propenoic acid 4-(nitroxy) butyl ester) was a kind gift from NicOx (Bresso, Milan, Italy) and Biolipox (Stockholm, Sweden). Stock solutions of 1 mM LTD4 were dissolved in 50% ethanol-water and then diluted in 20% ethanol-water. The concentration and purity of LTD4 was checked by UV spectroscopy. NCX 2057 and ferulic acid were dissolved in DMSO. OVA was dissolved in 0.9% NaCl. The other drugs were dissolved and diluted in Tyrode's solution or millipure water. Dilutions of drugs were freshly made from the stocks for each experiment. The drugs were present in the organ bath fluid during the remaining experiment. 5 μl of DMSO was added as a control and did not influence the baseline or cumulative contractions to OVA.

Analysis of nitrate and nitrite generation from SNP and NCX 2057

NCX 2057 (100 μM, n = 3) generated nitrite with a peak of 2280 ± 824 nM after 30 min. The nitrite generation was at one hour replaced by the generation of nitrate (1597 ± 345 nM) that continuously increased to 5729 ± 466 nM at two hours (fig 5). SNP (100 μM; n = 3) continuously released nitrite at around 800 nM for two hours in the organ bath (fig 5). The generation of nitrate from SNP was not possible to assess due to unspecific binding during analysis. The solvent DMSO (5 μl) or Tyrode's buffer did not generate any nitrite or nitrate (n = 3, fig 5).

Discussion

In this study we evaluated the effect of SNP and NCX 2057 during antigen-induced contractions in the peripheral lung. Our data indicated that the tested NO donors acted specifically on the release of different antigen-induced mediators and also differently as smooth muscle dilators. These data imply that depending on the nature of the NO donor, specific inhibition of antigen-induced contraction occurs and results in consistent inhibition of contraction in response to antigen.

Recent studies of antigen-induced airway constriction in the guinea pig lung indicate that histamine, CysLTs and several prostanoids, as PGD2 and TXA2, contribute to the antigen-induced airway constriction [20–22]. Histamine is released from granulars in mast cells and basophils and histamine is an important mediator of the early allergic airway response and airway inflammation in asthma [5]. Leukotrienes are biosynthesised de novo via 5-lipoxygenase from cell membrane phospholipids in mast cells, eosinophils, neutrophils, basophils and alveolar macrophages [26]. Prostanoids are generated de novo by cyclooxygenase (COX) enzymes in most cells [7]. However, PGD2 is mainly produced by mast cells and is known as a marker of mast cell activation [27] and associated with inflammatory conditions [28]. PGD2 is released in the absence of degranulation, and may be stored or rapidly synthesized and released in distinct pathways from degranulation after IgE ligation. TXA2 is mainly produced by platelets, macrophages and neutrophils [7]. TXA2 is increased in the airways of asthmatics after allergen challenge [6]. In the present study, measurements of histamine and eicosanoids in the bath fluid after challenges with antigen showed that histamine was released in highest amounts followed by prostanoids and then leukotrienes from the GPLP preparations. However, leukotrienes are known to be 1000 times more potent bronchoconstrictors than histamine and the induced contractions are more long lasting [29]. Interestingly, NCX 2057 and SNP had different inhibitory profile on the measured mediators in the present study. SNP blocked the release of histamine after challenge with OVA, whereas NCX 2057 reduced the synthesis of leukotrienes and prostanoids, suggesting that SNP and NCX 2057 may attenuate the activation of inflammatory cells and mediator release in distinctly different steps. NOx-containing molecules may react with oxygen, superoxides, water, nucleotides, metalloproteins, thiols, amines, and lipids to form products with biochemical actions [2] in the peripheral lung tissue. NO may also bind to iron-containing enzymes via nitrosylation and form iron-nitrosyl complexes, resulting in altered enzymatic activity and function [30]. S-nitrosylation is an important posttranslational modification that regulates NO transport and cell signalling, and thereby the activity of a vast number of proteins in different cell types, including mast cells and macrophages [30–32], and may be a crucial regulator of airway responsiveness [33]. There are implications that NO decrease leukotriene synthesis in mast cells [13] and macrophages [11]. A possible explanation of the inhibitory effects of NCX2057 in the present study, is therefore that this lipophilic NO donor has the potential to inhibit de novo synthesis of leukotrienes and prostanoids via interaction with iron centre in 5-lipoxygenase and cyclooxygenase enzymes [34, 35], and via interaction with thiol groups on LTC4 synthase [34]. In a previous study in the GPLP in vitro system, inhibition of NO synthases enhanced antigen-induced contractions by increasing the synthesis of CysLTs in the peripheral lung [14]. The effect of NCX 2057 on the antigen-induced response could also be explained by the fact that this molecule is derived from the parent compound ferulic acid, a substance described to have anti-inflammatory and anti-oxidant potentials [18, 36]. NCX 2057 has previously been implicated to have anti-inflammatory properties [17]. Nonetheless, in the present study the ferulic acid substance did not influence either the antigen-induced contractions or the release of pro-inflammatory mediators, suggesting that the inhibitory effect of NCX 2057 is due to its NO conjugation. The other tested NO donor, SNP, completely reduced the release of histamine after OVA challenge in this study. Topical administration of SNP has previously been shown to reduce histamine-induced plasma exudation both in guinea pig in vivo and human nasal airways [37] and previous findings in the rat mast cell confirm that both SNP and nitrite may inhibit the release of histamine [38, 39], suggesting that SNP has a specific mode of action on degranulation. SNP was over all less effective than NCX 2057 in this study. Previous studies in the GPLP in vitro system show that antagonists to the histamine receptor, such as cetirizine, has very little effect on the early allergic response in the peripheral lung [15]. This is in line with the results shown by SNP as a potential histamine inhibitor in these experiments.

SNP did not relax the lung parenchyma whereas higher concentrations of NCX 2057 (100 μM) induced relaxation of the smooth muscle tissue in the present study. These results are in contrast to the previous findings in guinea pig aorta and pulmonary artery where NCX 2057 induced vasodilation at much lower concentrations (nM) [15], supporting the hypothesis that the smooth muscle tissue in the peripheral lung is less sensitive to NO mediated dilations. The guanylyl cyclase inhibitor ODQ did not inhibit the relaxation induced by NCX 2057 despite the fact that 8-Bromo-cGMP induced strong relaxation in the GPLP system. This is in contrast to the cGMP-dependent dilation shown by NCX 2057 in vascular system in a previous study where ODQ effectively prevented the induced vasodilation [15]. Nevertheless, nitrovasodilators have previously been shown to induce smooth muscle relaxation both dependent and independent via cGMP activation [40–42].

Although the tested substances are referred as NO donors, the effect of SNP and NCX 2057 in the peripheral lung may not only depend on NO generation. The concentrations of the pharmacological agents were higher than are ordinarily used in vivo and also higher than NO levels produced in vivo. Measurements of nitrite and nitrate indicated that SNP and NCX 2057 differed notably in their kinetic profile. NCX 2057 produced significantly more nitrite than SNP at early timepoints (fig. 4). Nitrite accumulation is generally interpreted as an indicator of NO release. Thus, this could be a possible explanation for the decreased effectiveness of SNP relative to NCX 2057 as an inhibitor of lung contractility. However, it is important to realize that drugs may be metabolised to nitrate and nitrite without apparent formation of NO and therefore one cannot rely completely of nitrate/nitrite measurements as accurate estimates of NO formation. Several additional factors determine the rate of NO release and bioavailability of different NO donors including, lipophilicity, site of NO release, the presence of metal containing and thiol containing compounds, the stability of the NO bonds, and the propensity for homolytic vs. heterolytic cleavage [43]. In contrast to NCX 2057, SNP also transfers NO+ [43, 44], which can react with sulphydryl groups (-SH), to form S-nitrosothiols [45]. The mechanisms behind the distinct inhibitory effects of the tested NO donors on the early allergic airway response need to be further evaluated in other test systems.

Conclusion

The obtained results point at important differences between the tested NO donors, making it crucial to carefully characterize the profile of a selected NO donor, when using it as an experimental tool. The data revealed that the tested NO donors SNP and NCX 2057 acted as specific inhibitors of allergen-induced mediator release. In contrast to SNP, NCX 2057 also induced relaxation in the GPLP, but not via the cGMP pathway. The findings support that different NO donors may have specific anti-inflammatory effects in the peripheral lung tissue and may facilitate the development of anti-inflammatory therapeutic strategies targeting distinct effects of early allergic airway response.

Declarations

Acknowledgements

The authors are grateful for technical assistance by Margareta Andersson and Carina Nihlén. This work was financially supported by Swedish Heart Lung Foundation, Swedish Research Council of Medicine, Biolipox and Karolinska Institutet. Magnus Bäck was supported by the CMM-Söderberg Foundation.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

AKL conceived and participated in the design of the study, carried out the study, performed the statistical analysis, interpretation of data and drafted the manuscript. MB conceived and participated in the design of the study, interpretation of data and helped to draft the manuscript. JL participated in the design, interpretation of data and helped to draft the manuscript. SED conceived and participated in the design of the study, interpretation of data and helped to draft the manuscript. All authors read and approved the final manuscript.

Copyright

This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.